Aircraft gas turbine engines (as well as gas turbine engines for other applications) require auxiliary devices for their operation. Typical auxiliary devices include fuel pumps (oftentimes boost pumps and main pumps), hydraulic or lubrication pumps, electrical alternators or generators (which can include the FADEC [full authority digital electronic control] power source) standby generators, pumps and the like, and other such devices. It has been typical to provide a power takeoff shaft from the main engine to an auxiliary gearbox, and to drive all of the auxiliary units from the gearbox. This usually provides a very reliable system, but one which is not without its drawbacks.
One of the drawbacks is the fact that the auxiliaries are fixedly geared to the engine (and thus to each other) which requires (a) that all or most of the auxiliaries be driven when starting the engine, and (b) that the speed range of the auxiliary be rather extreme so they can produce useful output at starting rpm and rated output at a much higher running rpm.
With respect to engine starting, the auxiliaries for a commercial jet turbine engine often require hundreds of horsepower, and all of that machinery must be rotated by the equipment which is used to start the gas turbine engine. The demands on the power needed to direct the starter are obvious. More importantly is the occasional necessity to start the engine in an emergency situation (such as a windmill start after a flameout), and the added horsepower requirements for rotating the auxiliaries make the starting problem more difficult.
With respect to the operating range of the auxiliaries, those which are critical to engine starting, such as the fuel pump, and the FADEC alternator, must produce rated output at low starting rpm's in order to start and control the engine. For example, with respect to the FADEC alternator, only 8% of maximum rpm must produce enough power to keep the FADEC functional. That requirement results in a rather large and complex alternator which, at running rpm, produces far more power than is needed by the FADEC.
Similarly, with respect to the fuel pump, it must have the capacity to produce an adequate flow rate at starting rpm (again, 8% to 10% of maximum rpm) to start the engine. Thus, when cruising at altitude, the flow rate from the fuel pump is so high as to require a substantial bypass flow which is undesirable in that it consumes excess horsepower and uses that excess horsepower simply to heat the fuel. While the fuel can act as a heat sink in aircraft, the heat sink capacity of the fuel is limited particularly at altitude. Thus, the excess capacity of the fuel pump is more than wasteful, it sometimes heats the fuel beyond an upper desired temperature.
One further problem which results from utilizing the conventional mechanical drive for driving the auxiliaries is the fact that the auxiliaries must all be located on or very near the gearbox and that can adversely impact the aerodynamics of the engine envelope for aircraft.
It has been proposed to eliminate the power takeoff shaft and auxiliary gearbox, and replace them with a pneumatic takeoff from the main engine compressor which drives an auxiliary turbine which in turn drives all of the accessories. Griffith U.S. Pat. No. 2,612,020 describes such a system. The system proposed in that patent has the capability of alleviating the engine envelope problem in that greater flexibility for the placement of the auxiliaries can be provided. However, in the system described in that patent, all of the auxiliaries are driven from the auxiliary turbine, and the auxiliary turbine has a regulated speed, such that when the engine is running, the effect is not unlike the mechanically coupled auxiliaries of the conventional approach. All of the auxiliaries are operating at a fixed speed with respect to each other and to the engine. Furthermore, since some of the auxiliaries must be operated in order to start the engine, not only will all of the auxiliaries be operated, but all will be operated in the same speed ratios, whether or not they are needed in the starting operation. That produces an unnecessary load on the starting system much as is done with the conventional mechanically coupled auxiliary system.